The Legitimate Aims of Self-Defense


| Mar 4, 2021

Self Defense. 2020. Guided-missile destroyer USS Chafee (DDG 90) launches a Block V Tomahawk. U.S. Navy photo by Ens. Sean Ianno

The United States’ strike on Iranian-backed militias along the Syria-Iraq border on February 25 and the accompanying justifications, communications, and explanations have produced a flurry of legal discourse across the blogosphere and social media (here, here and here, for example).

Debates about the legality of the action under international law correctly focus on the application of the core criteria for using force in self-defense: the existence of an armed attack or imminent armed attack, and the necessity, proportionality, and immediacy of the use of force.

Within this broader framework, the U.S. strikes have raised two primary questions. First, did the actions of the non-State groups constitute one or more armed attacks on the United States so as to trigger the right to use force in self-defense? And second, was the U.S. strike in response a necessary and proportionate use of force?

On the latter question, much of the debate centers on whether the United States has adequately justified the use of force to “deter future attacks” and “respond to future threats,” as presented in the U.S. Article 51 letter to the United Nations Security Council. And on some level, the debate simply stalls at the point where some commentators consider that the U.S. goal in using force in self-defense—stopping further attacks in a campaign of attacks—was sufficient and others believe it was not in the absence of an ongoing or identifiable imminent next attack at that time. A deeper look at what are the legitimate aims of self-defense can help to shake loose some of this impasse and put a finer point on how necessity and proportionality guide the application of the law of self-defense.[1]

The classical recitation of the criteria for the lawful use of force in self-defense affirms that any such force must be necessary and proportionate—that is, the defensive act must be appropriate in relation to the ends sought in using force. Necessity addresses whether there are adequate non-forceful options to deter or repel the armed attack. Proportionality assesses the extent of the use of force against the overall military goals, such as deterring or repelling an attack. Applying both of these criteria depends on the victim State’s aim or objective in using force in response to the armed attack or imminent armed attack. In essence, it is not merely that the State needs to do “something” in acting in self-defense; the question is what the State needs to accomplish to defend itself. The parameters of the legitimate aims of self-defense are therefore essential: if we do not agree on what a State may or may not seek to achieve when acting in self-defense, we cannot effectively debate or determine whether using force was necessary or proportionate.

The fundamental premise of self-defense is that a State is not rendered helpless when faced with an attack, but rather can respond to protect its territory, sovereignty, nationals, and interests. The most basic and widely-supported aim of self-defense, therefore, is to halt or repel an attack, including imminent attacks in the context of anticipatory self-defense (although significant disagreement remains regarding what is an imminent attack and when the right of self-defense is actually triggered). In the context of terrorist groups and other actors who seek to attack in ways that cause significant harm but do not lead to direct engagement with the State’s forces, the notion of halting or repelling an attack is of limited application. As a result, prevention of imminent attacks is a common theme of strikes against terrorists and terrorist groups, such as the 2014 U.S. assertion of self-defense against the Khorasan Group in Syria or the 2015 U.K. assertion of individual self-defense against ISIS.

The U.S. communications about the February 25 strike do not rely on stopping an ongoing attack or preventing an identifiable imminent attack, but rather suggest that the United States acted to prevent additional, further, and/or future attacks by the non-State militia groups. The arguments about lawfulness thus are not really about whether force itself was necessary, instead of a non-forceful action. They are, more accurately, about whether the objective the United States sought to achieve with that use of force was an acceptable goal in self-defense. In other words, if force is the only way to achieve the objective, but the objective is not a valid aim in self-defense, then the self-defense claim would fail. Analyzing solely whether the force was necessary, or proportionate, however, obscures this underlying question.

The conservative or limited conception of the legitimate aims of self-defense—halting or repelling an attack—leaves States with minimal options in the context of terrorist attacks. Terrorist attackers either escape before or die during an attack, and the leaders remain far from the point of attack at all times. As a result, there is often no one for the State to repel at the moment of the attack. Similarly, insurgent groups often engage in periodic attacks rather than instigate a sustained engagement with State forces where they would be at a grave disadvantage. With both, superimposing the notion of attack and repel onto the counterterrorism or counterinsurgency context therefore leaves much to be desired.

So, what can States seek to accomplish in self-defense?

Although the extent to which international law allows a State to use force for goals beyond repelling or halting an ongoing attack or preventing an imminent attack is unclear—or certainly underexplored—a look at how States have characterized the purposes of their actions in self-defense is instructive.

States responding to attacks that have been completed commonly point to the need to act in self-defense against future attacks and future threats, as the United States asserted regarding the February 25 strike. In announcing strikes in response to the 1998 Embassy bombings, for example, President Clinton stated that “[w]ith compelling evidence that the bin Ladin network of terrorist groups was planning to mount further attacks against Americans and other freedom-loving people, I decided America must act.” The nature of terrorism suggests that preventing future attacks must be a legitimate aim in self-defense—the right of self-defense becomes a charade if a State must always absorb a terrorist attack rather than seek to prevent it.

But preventing future attacks is a remarkably elastic concept, particularly when terrorist or insurgent groups have the capability to launch attacks at targets far from their base of operations. More granularity is needed therefore—the justification of preventing future attacks does not yet offer sufficiently useful guidance on the extent to which a State may act in self-defense. To effectively analyze and apply necessity and proportionality instead depends on the goal of acting in self-defense, the objective the State seeks to achieve to prevent those future attacks.

Here, States’ declared goals in preventing future attacks fall along a spectrum, from a limited goal of ending an immediate campaign of ongoing and subsequent attacks to a much broader objective of defeating or destroying the group that launched the armed attack in order to prevent it from continuing to attack. Examples of action to end ongoing attacks includes Israel’s 2008-2009 military operation to “stop Hamas’ almost incessant rocket and mortar attacks” on Israel—the force was used to stop actual attacks and the continuing campaign of those attacks.

As one broader step, such goals can include eliminating or degrading the group’s capability to attack and dissuading it from future attacks by weakening its will to attack. For example, Turkey’s Operation Sun in 2008 was launched to “destroy PKK camps and hunt rebels of the PKK,” a purpose generally accepted as justifiable to weaken its adversary’s capabilities substantially.

The broadest conception of preventing future attacks includes action to “degrade and destroy” or to “disrupt, dismantle and ultimately defeat” an adversary group, the goals the United States has declared with respect to ISIS and al Qaeda, respectively, and the U.K. ultimately declared with respect to ISIS.

Determining that destroying or defeating a group is the most effective way to prevent future attacks is not unreasonable as a strategic matter, and may well be the only way to prevent further attacks, in which case it should be a legitimate aim in self-defense. It nonetheless presents substantial challenges from a legal perspective because of the difficulty in conceptualizing parameters for such a malleable concept and identifying when such a group is actually defeated. The U.S. use of self-defense to justify action against al Qaeda and a not-fully-enumerated cast of associated forces is, of course, the most obvious example of the consequences of an overly elastic conception of self-defense.

For last week’s strike, however, zeroing in on the aims of self-defense can help to advance the debate. Using force to show that the United States will not sit idly by in the face of attacks is one way to interpret “deter further attacks;” using force to degrade or neutralize specific capabilities used in planning and carrying out attacks is an entirely different way to understand that general statement. The former sounds more like retaliation and messaging through force, while the latter reads as specific action to render the group incapable of its next intended acts.

The United States obviously benefits from the flexibility that less precise language here offers, but the generalities also fuel the interpretation that the United States lacked sufficient basis for its action. Rather than let necessity and proportionality masquerade as criteria in an argument that is really about the appropriate objectives of self-defense here, a deeper examination of the legitimate aims of self-defense will mean that necessity and proportionality can really be put to work in assessing the lawfulness of the use of force.


Laurie R. Blank is a Clinical Professor of Law, Director of the Center for International and Comparative Law and Director of the International Humanitarian Law Clinic at Emory University School of Law.



[1] This post is based on a longer article of mine, The Extent of Self-Defense Against Terrorist Groups: For How Long and How Far?